The use of chlorine in swimming pools to control the growth of algae and to purify the pool water is well-known. Because chlorine gas is very toxic and hazardous to store, chlorine has been added to pools in various chemical forms. Concentrated solutions of chlorine dissolved in water or other liquid has been the most common form of adding chlorine to a pool. The liquid chlorine is heavy to transport and store, and has very limited shelf life. Chlorine is also sold in the form of granular material which can be added to the pool water and is highly soluble. Chlorine is also available in cake or tablet form which is designed to dissolve at a controlled rate to release chlorine into the pool water over an extended period of time.
Automatic chlorinators using the cake form of chlorine have been used which flow water over the chlorine cake at a controlled rate while the pool pump is running. The dissolved chlorine is then mixed with the pool water. Adding chlorine in this manner has a number of disadvantages, the cake form is relatively expensive, it is strongly on the acid side, and the rate of adding the chlorine is difficult to control. The granular form of chlorine is preferable in that it is less expensive, its acid-alkalinity balance is neutral, and it is easy to store. However, the granular form of chlorine has not been utilized satisfactorily for automatic chlorinating systems. One such system is described in U.S. Pat. No. 3,626,972. In the arrangement shown in the patent, the granular material is stored in a supply bin from which it is released by gravity to introduce a measured volume of granules into a measured volume of water each time the pump cycles. One of the problems with granular dispensers of the type disclosed in the prior art is that the granular chlorine in the presence of moisture insufficient to dissolve the granules tends to swell and harden into a crusty form which does not then feed out of the storage container properly. Moreover, the granular chlorine is mixed in a region open to atmosphere, allowing chlorine gas to escape with its corrosive and toxic consequences.